Electron tubes are, of course, very well known, and include the species of traveling wave tubes, in which a stream of electrons is focused or directed into an elongated beam that passes through a coil carrying r-f energy. The r-f energy in the coil interacts with the stream of electrons to cause bunching of the electrons along the stream. The bunches, in turn, interact with further turns of the coil to amplify the r-f energy in the coil.
Such traveling wave tubes are in very common use in the electronic world. However, they have the inevitable disadvantages of the bulk necessary to accommodate the elongated electron beam and the need for separate electrical or magnetic focusing system. Such a tube would also require a relatively high voltage between the cathode and anode to maintain the electron stream.
The search for small size, lightweight, high-efficiency, and high-reliability microwave devices turns to solid state devices, but these devices are currently limited, in peak and average power, at microwave frequencies. To overcome the peak power limitations of solid state devices, electron-beam semiconductor (EBS) devices are being developed. In those devices, a modulated electron beam strikes a semiconductor target comprising a reverse-biased p-n junction. Each incident electron striking the semiconductor produces thousands of carrier pairs in the junction region, with a correspondingly large current gain. The semiconductor is connected through an external, r-f, lumped output circuit to utilize this current gain.
The main limitation of the electron-beam semiconductor devices is the inevitable transit time through the semiconductor and the external circuit. Transit time effects and stray capacitance loading are such that there is an upper frequency limitation of approximately three gigahertz.